Refactor Alias cop
Add new cop Not
Simplify a bit the ClassMethods cop
Add new cop Eval
Revert "Use sensible default configuration"

This reverts commit 25398336579b48c17cd8e812cacf71f7461b668c.
Fix broken specs caused by calling #for_cop for Hash instance
Fix SymbolArray cop is not properly disabled by default

This fixes #137.
Merge pull request #141 from yujinakayama/fix-symbolarray-not-disabled-default

Fix SymbolArray cop is not disabled by default
Add --only option for running a single cop.

This is very useful for debugging, and probably good for end users
as well.